1. Field of the Invention
The present invention relates to a turbine nozzle support structure for supporting the ceramic turbine nozzle of a gas turbine by pressing the turbine nozzle against a nozzle support.
2. Description of the Related Art
Increase of turbine nozzle outlet temperature, namely, turbine inlet temperature, in a gas turbine is effective in improving the efficiency of the gas turbine. A single metal blade or a segment blade formed by integrally combining a plurality of metal blades is used generally as a turbine nozzle for conventional medium and large gas turbines. Since the heat resistance capacity of the metal turbine nozzle limits the increase of the turbine inlet temperature, the turbine nozzle is cooled by compressed air produced by the gas turbine. The compressed air used for cooling the turbine nozzle does not contribute to the combustion of the gas, but decreases the gas temperature after combustion.
Consequently, the combustor outlet temperature needs to be further increased to maintain a predetermined turbine inlet temperature. The increase of the combustor outlet temperature increases NOx produced by combustion. The nozzle blade needs to be provided with a complicated cooling structure and to be formed of a heat-resistant material by precision casting. Therefore, the nozzle blade is expensive and is subject to deterioration due to oxidation and corrosion at high temperatures and is liable to be abraded due to decrease in hardness.
Application of ceramic materials resistant to oxidation, corrosion and abrasion at elevated temperatures to parts, which are exposed to high temperatures, of combustors and turbines has been tried in an attempt to solve those problems in increasing turbine inlet temperature. Gas turbines provided with ceramic parts to be exposed to elevated temperatures, as compared with turbines provided with metal parts to be exposed to elevated temperatures, are able to operate at higher turbine inlet temperatures without requiring cooling or using reduced cooling air, and are able to operate at higher efficiency, cause less environmental pollution, and have longer effective life.
However, ceramic materials, as compared with metals, are brittle and less elastic. If a ceramic part interferes with an adjacent metal part, the ceramic parts will be easily damaged and therefore considerable contrivance is needed to apply ceramic materials to the parts of combustors and turbines. Some medium and large gas turbines are provided with a turbine nozzle having a plurality of ceramic nozzle segments arranged in a circumferential arrangement to solve problems resulting from increase in thermal stress due to increase in their dimensions and problems related with manufacturing techniques. A support structure disclosed in JP-A 2001-317577 for supporting such nozzle segments presses the nozzle segments against support members with coil springs to absorb the difference in thermal expansion between the ceramic nozzle segments and the adjacent metal support members by the coil springs to prevent troubles due to the respective different modes of deformation of the ceramic nozzle segments and the metal support members.
However, only a limited space is available for installing a mechanism for holding the nozzle segments and hence the coil springs must be small. The holding capacity of the small coil springs is insufficient for holding the turbine nozzle consisting of the nozzle segments.